The present invention relates to a pneumatic radial tire, and more particularly to a pneumatic radial tire which reduces one-side drifting in driving and can be preferably used for passenger cars.
Radial tires which can improve steering stability, comfortable riding performance and so on are widely used. Tires which are superior in straight-forward driving performance by preventing one-side drifting of a car in order to increase the driving safety of a car are being demanded. Conventionally, one-side drifting is considered to be caused by the so-called conicity in which the circumferential lengths of the belt layer in the right and left sides in a tire are different from each other. Therefore, various methods have been taken in order improve the homogeneity in the right and left directions of the tire's axis.
On the other hand, due to the recent progress in tire measurement techniques, as shown schematically in FIG. 9, a cornering force, i.e. a lateral force F which is generated the lateral direction Y of a tire when a tiny slip angle (.alpha.) is given in the running direction X of the tire and a self-aligning torque SAT which revolves in the direction of the slip angle (.alpha.) about the vertical axis Z that passes the center of a tire can be measured at a high precision.
Such measurement results are shown using curve K in FIG. 10, by plotting the self-aligning torque SAT on the abscissa axis and the lateral torque F on the ordinate axis. In the curve K, the cases when the slip angle (.alpha.) is 0 deg., +0.1 deg. or -0.1 deg. are shown with a dot.
In such relation of self-aligning torque SAT and lateral force F, the lateral force F at the crossing point K1 of the curve K and the axis of ordinates, that is, the lateral force F when the self-aligning torque SAT is not generated, is called a residue CF. It was found that the residue CF is a tire characteristic which affects the one-side drifting of a car. In other words, a car is drifted to one side in the right direction when the residue CF is in the plus direction, i.e. the right direction. Thus, the one-side drifting characteristic of a car can be evaluated by the direction and the size of the residue CF. In order to prevent the one-side drifting of a car, it is required to reduce the residue CF.
The residue CF is generated from the expansion and contraction of the belt at the ground contact part. A shearing strain in a surface is created in the cross ply belt of a radial tire by the parallel movement of the cords due to the expansion and contraction. Thus, the tread rubber generates a steering torque by a shearing strain in a surface generated together with the strain of the belt ply in the outermost layer. It is considered that the lateral force F is created by this steering torque. Thus, it was found that the residue CF is caused by a belt and depends on the cord quantity of the belt and the inclination of the belt cords.
The cord quantity is defined as N.times.S, which is the product (in mm.sup.2) of the total cross-sectional area of one belt cord S (sq.mm) and the number of belt cords N laid in 10 cm in a right-angled direction to the belt cords. In other words, the cord quantity is the total cross-sectional area (mm.sup.2) of the belt cords N per 10 cm width of the belt ply. The rigidity of the belt can be reduced by reducing the cord quantity N.times.S and enlarging the inclination angle of the cords to the direction of the tire's equator. It is known that this reduces the hooping effect of the belt and then the residue CF, thereby controlling the one-side drifting of a car.
On the other hand, such reduction of the rigidity of the belt can improve the comfortable riding performance at the same time, which is a basic item required for a car.
Additional experiments were conducted about the reduction of the residue CF in a tread part having a relatively low belt rigidity. As a result, it was found that good results could be obtained by reducing the inclination of the lateral grooves crossing the circumferential grooves, that is, constructing the lateral grooves closely in the direction of the tire's axis. However, such a tread pattern does not appeal to customers, because it lacks an aesthetic sense, and tends to decrease the marketability of the product.